The present invention generally relates to communications systems wherein signal information is processed in analog forms. More specifically, the invention relates to systems and methods for tuning resistance-capacitance (xe2x80x9cRCxe2x80x9d) continuous-time filters.
RC continuous-time filters are utilized in many applications, including communications systems. As with other types of filters, RC continuous-time filters may be designed to selectively filter out parts of a signal that have frequencies above or below a desired cut-off frequency. Typically, RC continuous-time filters are utilized in applications where the signals are expected to remain continuous in time and have continuous or analog levels. Since continuous-time filters can typically be utilized without the need for signal sampling, continuous-time filters provide a significant operating-speed advantage over the switched-capacitor filter counterparts.
One example of a popular application in which RC continuous-time filters are utilized is digital subscriber line (xe2x80x9cDSLxe2x80x9d) communications systems. DSL communications systems have been introduced and implemented by communications systems providers in recent years to provide customers with a wide variety of interactive multi-media communications signals over existing plain old telephone system (xe2x80x9cPOTSxe2x80x9d) communications lines. As is known in the art, a typical DSL communications system may include, among other components, an analog front end (xe2x80x9cAFExe2x80x9d).
In practice, generally, a digital transmit signal may be provided to an AFE, where the AFE converts the digital signal to analog for transmission along the POTS system. The AFE may also contain a plurality of filters used to eliminate any out of band noise that may have been generated by preceding stages in the system or during the digital to analog conversion. The. analog transmit signal is then sent to a line driver to amplify the signal for transmission. In the receive path, the AFE may convert an analog receive signal into digital and, likewise, properly filter out any out-of-band noise. In some instances, the filters may be analog filters, and in others digital filters.
Within the AFE of a DSL communications system, one or more RC continuous-time filters may be utilized to perform the filtering mentioned above, as well as possibly for other purposes. Typically, the components of a DSL system are contained in a common location. As a result of this practice, DSL system components, including RC continuous-time filters integrated within the AFE, may be subject to temperature variations affected by ambient temperature variations. For example, in typical applications of DSL communications systems, operating temperatures of system components may vary from xe2x88x9240xc2x0 C. to 125xc2x0 C. due to ambient temperature and/or operating condition variations.
It is noted that the preceding discussion, with respect to DSL communications systems, merely presents one example of the utilization of RC continuous-time filters, and there are many other applications in which RC continuous-time filters are or may be utilized and in which temperature variation may be a concern.
Although RC continuous-time filters offer an operating-speed advantage, as discussed above, these filters typically require some means of tuning in order to set and maintain a desired cut-off frequency. One reason that tuning of RC continuous-time filters is desired is because the cut-off frequency of the filter is dependent on the values of the resistance and capacitance elements of the filter, and these values will typically vary due to temperature variations. For example, in a filter built with xe2x80x9cHigh-resitivity Poly0xe2x80x9d resistors and xe2x80x9cInterpolyxe2x80x9d capacitors (note, these component types are known in the art and have certain temperature variation characteristics), the variation in cut-off frequency may vary by as much as 0.15%/xc2x0 C. In contrast, an acceptable range of cut-off frequency variation, depending on the application, is typically less than 0.04%/xc2x0 C. It is noted that in an RC continuous-time filter that is built with High-resitivity Poly0 resistors and Interpoly capacitors, the temperature variation of the RC component of the filter, and thus the cut-off frequency of the filter, is typically dominated by the temperature variation of the resistors which can be as large as 0.15% per xc2x0 C.
Therefore, there is a need for improved methods, systems and devices for tuning RC continuous-time filters in order to set and maintain cut-off frequencies within acceptable tolerances with regard to temperature variations.
The present invention relates to improved methods, systems, and devices for tuning RC continuous-time filters in order to set and maintain cut-off frequencies with regard to temperature variations. A representative method for tuning an RC continuous-time filter includes: varying the resistance of a coarse-tuned resistive element coupled to an input of the filter for varying the cut-off frequency of the filter based upon process variations; and continuously adjusting the resistance of a MOSFET transistor in series with the resistive element, such that the adjustment in the resistance of the transistor offsets the adjustment in the resistance of the resistive element, due to temperature variations.
In another embodiment, a system for tuning an RC continuous-time filter includes a coarse-tuned resistive element coupled to an input of the filter for varying the cut-off frequency of the filter based upon process variations. The system also includes a MOSFET transistor coupled to the coarse-tuned resistive element, wherein the MOSFET transistor provides a resistance dependent upon a voltage offset provided to the gate of the transistor. The resistance of the MOSFET transistor offsets an adjustment in the resistance of the coarse-tuned resistive element, due to temperature variations. Also included in the system is a voltage offset generator configured to provide the voltage offset to the MOSFET transistor.
Another embodiment of the invention may be construed as an automatically-tunable RC integrator. The integrator includes a coarse-tuned resistive element coupled to an input of the integrator for varying the cut-off frequency of the integrator based upon process variations. The integrator also includes a MOSFET transistor coupled to the coarse-tuned resistive element. The MOSFET transistor provides a resistance dependent upon a voltage offset provided to the gate of the MOSFET transistor, wherein the resistance of the MOSFET transistor offsets an adjustment in the resistance of the coarse-tuned resistive element due to temperature variations.
In yet another embodiment, a system for tuning an RC integrator includes means for varying the resistance of a coarse-tuned resistive element coupled to an input of the integrator for varying the cut-off frequency of the integrator based upon process variations. The system also includes means for continuously adjusting the resistance of a MOSFET transistor in series with the coarse-tuned resistive element, such that the adjustment in the resistance of the MOSFET transistor offsets the adjustment in the resistance of the coarse-tuned resistive element, due to temperature variations.